1. Field of the Invention
The present invention relates to a method for adaptive triggering of A breathing device as well as to a breathing device operating according to the method.
2. Description of the Prior Art
“Triggering” as used herein means the activation of any respiration phase, i.e. both inspiration phases and expiration phases. This is a broader meaning than that normally understood by the term (triggering is normally only related to the activation of inspiration phases).
“Breathing device” as used herein means all known devices providing a breathable gas to a subject. This includes, inter alia, ventilators, respirators, anaesthetic machines and resuscitation devices.
State of the art breathing devices have a triggering functionality based on the gas parameters flow and pressure.
A flow triggering system is known from European Application 0 459 647, which discloses a breathing ventilator where a predetermined rate of flow of gas is delivered toward a patient. Changes in the rate of flow are measured and a breath support is triggered when the change in the rate of flow exceeds a threshold value (trigger level).
A pressure triggering system is known from U.S. Pat. No. 4,050,458 wherein pressure is measured and is analyzed with respect to the change of sign of a differentiated pressure signal. When a change occurs, an assisted inspiration phase can be started. In order to avoid self-triggering due to naturally occurring variations in pressure, the presence of a predetermined drop in pressure can be an additional requirement for triggering the inspiration phase.
Although these systems normally operate satisfactorily, there will be a delay time from the actual onset of a spontaneous inhalation attempt (originating in the respiratory center of the brain) until triggering actually occurs. This delay time may be more than 200 ms. Part of this is due to the transit time of the nerve signal and the response time of respiratory muscles, which have to start working before a change in pressure and flow can occur. The primary part of the delay, however, is due to the fact that triggering levels are set high enough to avoid any risk of self-triggering (i.e. the device being triggered to start an inspiration phase when there is no attempt made by the patient). It thus takes time before the effects of an inhalation reach the trigger requirement and start an inspiration phase.
This delay time is also present for variants of the flow and pressure trigger systems, such as volume trigger systems.
One attempt to avoid or reduce the delay time is disclosed in U.S. Pat. No. 5,373,842, wherein a pressure trigger system utilizes flow measurements on a bias flow to change the required trigger pressure level.
Although this result is a more stable trigger system with shorter response time, some of the delay time nevertheless remains.
Other known trigger systems use other parameters such as impedance across the chest disclosed in European Application 0 324 275, nerve signals as disclosed in PCT Application WO 00/00245 and muscle (myoelectric) signals as disclosed in PCT Application WO 99/43374.
The first of these essentially have the same delay times as the flow/pressure related triggering parameters, since the impedance will not change until the lungs start changing due to muscle activity. Here, also, thresholds must be set to avoid self-triggering from other impedance sources.
The latter two have less delay, but are not ideal in all situations. Muscle detection, for instance, normally relates to myoelectrical signals in the diaphragm. As stated in PCT Application WO 99/43374, however, inhalations can start with other muscle groups. Measuring activity in all muscles related to respiration is not realistic. A solution to this problem is suggested in PCT Application WO 99/43374, namely to have a separate flow or pressure trigger system operating in parallel and use a first come, first serve trigger operation. The delay time then remains for the flow/pressure trigger system (as well as for the muscle trigger).
It should also be noted that all systems triggering on excitable cell signals (nerves and muscles) are at risk of self-triggering unless a sufficiently high threshold for the triggering is set.